I’m far from an expert, but I know the magnetic field is the result its molten core, so this is the first I’ve ever heard about Earth “surviving millions of years without a magnetic field.”
If it somehow lost its magnetic field, Earth would be subject to intense solar radiation, so life on this planet would end very quickly and dramatically,
However, the process of Earth losing its magnetic field itself would be a very slow process, I’d think. Starfield is set 300 years in the future, and I’m not sure if that’s a realistic timescale for that to occur.
As to why Earth lost its atmosphere in Starfield, there is a practical purpose for that: you can leave it barren and don’t need to populate it with people and cities, because an inhabited Earth 300 years in the future should be heavily developed.
